3.1.6a Chemical equilibria, Le Chatelier’s principle and Kc Quiz: AQA Chemistry, Unit 1

The 20 questions

1. What does a study of equilibria indicate about a chemical reaction, in contrast with kinetics?

   • The total energy that is released or absorbed when the reaction takes place
   • The activation energy that must be supplied before the reaction can start
   • How quickly the reaction will occur once the reactants are mixed together
   • How far the reaction will go before it reaches a balance of reactants and products

2. Which feature defines a reversible reaction at equilibrium?

   • The concentrations of reactants and products are always equal to one another
   • The forward and reverse reactions proceed at equal rates
   • The reaction has stopped, so no molecules are reacting in either direction
   • All reactants are completely converted into products with no reactants remaining

3. At equilibrium in a closed system, which quantity remains constant at fixed temperature?

   • The rate of the forward reaction only, which stays at zero
   • The concentrations of the reactants and products
   • The concentrations of all species, which fall to zero
   • The mass of the catalyst, which becomes zero at equilibrium

4. What does the symbol for a reversible reaction, a pair of opposing arrows, indicate?

   • The reaction can proceed in both the forward and the reverse direction
   • The reaction has gone to completion and no further change can occur
   • The reaction requires a catalyst in both the forward and the reverse direction
   • The reaction is exothermic in both the forward and the reverse direction

5. Why is the study of equilibria important in industry?

   • It determines the colour of the final products of the reaction in the reactor
   • It shows which reactions are spontaneous at absolute zero temperature in the plant
   • It predicts the yield that can be obtained from a reversible reaction under given conditions
   • It shows the atomic structure of the reactants that are fed into the process

6. Which description of dynamic equilibrium is correct?

   • Reactant and product concentrations are equal to each other in a balanced equilibrium mixture
   • Only the forward reaction continues at a constant rate until all of the reactants are used
   • Reactions have stopped completely, so the mixture is static and unchanging at a molecular level
   • Reactions continue in both directions at equal rates, so observable properties stay constant

7. In a reaction at equilibrium, the forward rate is 4.0 x 10^-3 mol dm^-3 s^-1. What is the reverse rate?

   • 0 mol dm^-3 s^-1
   • 8.0 x 10^-3 mol dm^-3 s^-1
   • 4.0 x 10^-3 mol dm^-3 s^-1
   • 2.0 x 10^-3 mol dm^-3 s^-1

8. Which process gives a yield very close to 100% and so is not an equilibrium system?

   • The Haber process for ammonia at 450 C and 200 atm
   • Esterification of ethanol with ethanoic acid in the presence of an acid catalyst
   • Complete combustion of methane in excess oxygen
   • Dissolving ammonia gas in water to form an aqueous solution of ammonia

9. Which of the following is a reversible reaction?

   • Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g) in an open beaker
   • N2(g) + 3H2(g) <=> 2NH3(g)
   • CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
   • 2Mg(s) + O2(g) -> 2MgO(s)

10. A student mixes reactants in a closed container and finds that the concentrations stop changing after a time. What does this show?

    • All of the reactants have been converted into products, leaving none behind
    • The reaction has stopped completely, so no molecules are reacting in any direction
    • The catalyst has been used up, so the reaction cannot continue to form products
    • The system has reached equilibrium, with forward and reverse rates equal

11. Which of these changes reaches equilibrium faster without changing its position?

    • Adding more of one of the reactants to the mixture
    • Adding a catalyst to the mixture
    • Raising the temperature of an exothermic reversible reaction
    • Removing one of the products as it forms in the mixture

12. Two identical vessels contain the same reactants. One has a catalyst and reaches equilibrium in 1 minute, the other without catalyst in 2 minutes. How do the equilibrium yields compare?

    • The uncatalysed vessel has the higher yield, because the catalyst slows the reverse reaction
    • The yield depends on the mass of catalyst added to the vessel, not on the conditions
    • The catalysed vessel has the higher yield, because the catalyst makes more product form
    • The yields are the same, because a catalyst changes only the rate of reaching equilibrium

13. In a closed system at equilibrium at constant temperature, which observable property does not change?

    • The total number of molecules, which changes steadily with time
    • The colour of the mixture, which keeps changing continuously
    • The forward rate only, while the reverse rate keeps changing
    • The pressure and the colour of the mixture

14. Which pair describes a reversible reaction taking place in aqueous solution?

    • Na(s) + H2O(l) -> NaOH(aq) + H2(g)
    • CH3COOH(aq) + H2O(l) <=> CH3COO-(aq) + H3O+(aq)
    • AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)
    • Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

15. For the simple reaction A <=> B, both steps are first order and the forward rate is kf[A] and the reverse rate is kr[B]. What is the equilibrium ratio [B]/[A]?

    • [B]/[A] = kf/kr
    • [B]/[A] = kf - kr
    • [B]/[A] = kr/kf
    • [B]/[A] = kf x kr

16. For the reaction 2A <=> B, the forward rate is kf[A]^2 and the reverse rate is kr[B]. At equilibrium [A] = 0.5 and [B] = 0.2 mol dm^-3, with kf = 0.4. What is kr?

    • 0.1
    • 0.5
    • 0.2
    • 2.0

17. Why is a closed system needed to define equilibrium?

    • A closed system raises the temperature, so the reaction always reaches a higher yield
    • A closed system stops molecules from colliding, so the rates of reaction become zero
    • In an open system reactants or products can escape, so the balance of rates cannot be maintained
    • In an open system the catalyst is destroyed, so equilibrium is never reached at all

18. A reaction at equilibrium is cooled and the concentrations change until a new balance is reached. What does this show?

    • The equilibrium was never real, because the concentrations of the species could still change
    • The equilibrium constant has been destroyed by the cooling, so no equilibrium can exist
    • The equilibrium position depends on conditions, so a new equilibrium forms at different concentrations
    • The rate of the reaction has fallen to zero and the reaction has permanently stopped occurring

19. Which statement about the approach to equilibrium from either direction is correct for a reversible reaction at constant temperature?

    • Starting with products gives a higher equilibrium yield of reactants, because products favour the reverse direction
    • Starting with only reactants means no equilibrium can ever form, because products are absent
    • Starting with only products always gives a different equilibrium constant from starting with reactants
    • The same equilibrium mixture is reached whether the start is all reactants or all products

20. A reversible reaction has reached a state where its forward and reverse rates are equal. What is true of the net change in the concentration of each species at that point?

    • The net change in concentration of each species is zero
    • Reactant concentrations increase while product concentrations fall
    • Each species concentration is increasing at the same steady rate
    • All species concentrations are decreasing steadily towards zero

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